1. Field of the Invention
This invention relates to a process control system, and more particularly to a process control system using an improved Smith's method for dead time compensation.
2. Discussion of the Background
Control units processing PI or PID adjustment functions have been widely used in all industrial fields throughout the history of process control, and are still indispensable to plant operation.
When a process system is approximated by the dead time L and first order lag T (a time constant), the process system can be simply controlled by PID control in the case of first order lag alone. However, when dead time L is included, as dead time L becomes greater, in other words as L/T becomes greater, control by PID control alone gradually becomes more difficult.
Therefore, as a method for improving the controllability of a process system which includes dead time, O. J. M. Smith (personal name) proposed the so-called `Smith method` or `dead time Smith compensation method` which is now widely used. This is designed to control the apparent first order lag process system alone by adding a dead time compensator, which uses a process system model, to PID control and shifting dead time outside the control loop.
FIG. 1(a) shows a function block diagram of a control unit which uses such a dead time Smith compensation method. In this unit, dead time compensator 5 is newly added to a control system. Deviation computing device 1 obtains the deviation En, between target value SVn and controlled variable PVn. PID adjustment device 3 performs PI or PID control operation based on this deviation En, and impresses manipulating signal MVn obtained on process system 2.
In dead time compensator 5, first order lag model device 6 outputs manipulating variable MVn of PID adjustment device 3 through a first order lag transfer function. Process system model device (also called process system model`) 7 outputs the same manipulating signal MVn of PID adjustment device 3 through a transfer function with first order lag and dead time. Subtraction device 8 subtracts the output of process system model 7 from the output of first order lag model device 6.
The construction is such that the output of subtraction device 8 is conducted to subtraction device 4, which is provided on the output side of deviation computing device 1, where the output of dead time compensator 5 is subtracted from deviation En.
Here, in FIG. 1(a),
Gp.multidot.e-Lp.multidot.s: Transfer function of the process system PA1 Gp-Kp/(1+Tp.multidot.s): Transfer function of process system with dead time eliminated PA1 Lp: Process system dead time PA1 Kp: Process system gain PA1 Tp: Process system time constant PA1 s: Laplace operator PA1 Gm.multidot.e-Lms: Process system mode transfer function PA1 Gm=Km/(1+Tm.multidot.s): Transfer function of process system model with dead time eliminated PA1 Lm: Process system model dead time PA1 Km: Process system model gain PA1 Tm: Process system model time constant,
also,
and when FIG. 1(a) is rearranged by equivalent conversion, it becomes as in FIG. 1(b).
Here, disturbance D is small and can be ignored. Also, if the condition of agreement of the process system characteristic and the characteristic of process system model 7 is assumed, that is to say that there is the relationship EQU Disturbance=small, Tp=Tm, Lp=Lm (1)
we have Gp=Gm, when the transfer constant for Svn-PVn in this case is found, it becomes EQU PVn/SVn={(Gc.multidot.Gm)/(1+Gc.multidot.Gm)}E-Lp.multidot.s (2)
and it can be converted to the type of construction in FIG. 1(c). Therefore, this means that, in this control unit, first order lag model device 6 with the dead time eliminated may be feed-back controlled by PID adjustment device 3. In other words, since dead time can be removed from the control loop, this unit can easily be controlled by PID adjustement device 3, and good controllability can be expected. Dead time element 9 is placed outside the control loop.
However, as is evident from the above explanation, control units using the above type of dead time Smith compensation method cannot be constructed as shown in FIG. 1(c) unless the conditions of Equation (1) are established.
Nevertheless, it is difficult always to establish the conditions in Equation (1) in actual plant control. For example, the conditions in Equation (1) may be altered in the course of time by variations in the characteristics of the process system and environmental changes, such as ambient temperature, catalyst temperature, raw material conditions or size of load. As a result, the more the conditions in Equation (1) vary, the more the controllability deteriorates. Thus, there is the problem of becoming unable to achieve the functions of the dead time Smith compensation method, resulting in a great influence on the controllability of the plant.